This invention relates generally to a gusseted bag of packaging material and specifically to a side-gusseted bag of thermoplastic tubular film.
It is conventional to make bags from tubular theromoplastic film. Tubular film is made by coextruding a multilayer film by the conventional blown bubble method. The nascent extruded tube is then cooled, collapsed, and fed through a hot water bath where the tube is sufficiently softened so that it can be stretch-oriented by the trapped bubble technique. The oriented bubble is then cooled, collapsed, and wound up as lay-flat, seamless tubular film to be used later to make bags. For example, end-seal bags are typically made in series by transversely heat sealing across the width of lay-flat tubing at regular intervals followed by severing the tubing so that the transverse seal forms the bag bottom and the collapsed edges of the tubing form the bag sides.
In certain packaging applications, it is desirable to utilize gusseted bags, for example bags having a gusset fold extending down each side. It is commercially advantageous to make gusset folds directly on seamless tubing in the bag making operation discussed above. For example, U.S. Pat. No. 3,485,145 for "Apparatus for Forming Gusseted Tubing" issued Dec. 23, 1969 to Jones discloses apparatus for producing gusset folds in a moving length of plastic tubing. A gusset former rides inside a bubble of advancing tubing and acts in cooperation with external guides to form a pair of gusset folds along diametrically opposed sides of the tubing.
Certain deficiencies have been observed in bags made from gusset-folded tubular film. It is difficult to produce a dead fold at the inside fold of a gusset on plastic film, which tends to hinder achieving uniformity in gusseted bag production. Another problem concerns integrity of the end-seal forming the bottom of such bags, particularly when made of heat shrinkable (oriented) film. Since the gusset folds extend to the bag bottom, the end heat seal tends to be irregular where the heat seal extends across the gusset folds. When using oriented film and upon heat shrinkage of the bag about a contained product, shrink tension tends to break the end seal.
It is therefore a primary object of the invention to provide a gusseted bag of thermoplastic film, especially tubular film, which has side gussets incorporating a dead fold. It is a secondary object to provide such a bag that incorporates a seamless bag bottom.